Warning! Firmware 18.21 bricked my 1040 with endless reboots.

You decided to become a beta tester without a copy of the settings/apps/records/achievements/courses. What could have gone wrong :)

For future betatesters - first make a copy of your data, then have fun testing non-production versions.

thanks for your snarky response. it was not very helpful. I also did not knowingly join the beta program.

Please post this in the beta forum. You might get better response from fellow beta testers.

What kind of help did you expect?
You don't backup your device before installing beta software.
You don't even know that you joined the beta program (sic!)
You are writing about this in the wrong section of the forum - there is a separate section dedicated to test software.
But that's me being malicious :) Put more cons next to my answers if it makes you feel better heh :)

See this for info on how to enroll and disenroll in the beta program:
https://forums.garmin.com/beta-program/w/beta-wiki/112/how-to-enroll-for-public-beta-program

There is already a discussion on this issue in the beta forum. You can click the Beta Program link at the top of this page. If you post there, someone may be able to help you to roll back your device to the current release.

And there is already a 18.23 which looks unchanged from 18.21 and which smells like a release candidate, notwithstanding there being possibly unresolved beta issues?

The OP did no more than post a warning to other users. He didn't complain about the beta, nor did he ask for any help making things good. Your unnecessarily aggressive responses have all the hallmarks of the knee-jerk beta police.

thanks!